1. Field of the Invention
This invention relates to communications systems, and more particularly to systems and methods for synchronizing digital bits in a data stream.
2. Description of Related Art
Typical digital communication systems involve using a transmitter to send a Abit stream to a receiver. The bit stream contains digital information that the receiver decodes and makes use of. In some communications systems, the digital information is extracted by first converting the analog representation of the bit stream to digital samples. Each sample represents the amplitude of the signal at the time of sampling. The digital samples may be analyzed for location of the bit transitions by detecting where the amplitude levels change.
Because of noise and other effects, the transitions may not be cleanly represented as digital samples. This is especially true in wireless communications systems having an air interface. Global Navigation Satellite Systems (GNSS) are especially sensitive because the GNSS signals are communicated between satellites above the earth's atmosphere and receivers on the earth's surface. In addition, the signal transmitted to the receiver is generated by the transmitter and therefore synchronized to the time base in the transmitter. Thus, the signal is not synchronized to the receiver's time base such that the location of the bit transitions in the receiver's time base cannot be assumed.
Several methods have been developed for detecting bit transitions in a stream of digital samples. Examples of such methods (from the realm of GNSS receivers) include:                1. “Histogram Method”—This method is based on 1 msec samples. In this method, the samples are analyzed for transitions in level between successive samples. The method incorporates heuristics based on counting transitions used to identify data bit edges. The problem with this method is that it fails below about 30 dB-Hz.        2. “Viterbi” method—This method is loosely based on an algorithm published by Andrew Viterbi. In this method, a trellis is used to track bit transition decision and a maximum likelihood method is used to determine data bid edges. The performance of this method is good. But, at low levels of carrier to noise spectral density ratio C/N0, the method yield unacceptable levels of “False Positive” indications—i.e., it falsely claims to have identified a data bit edge.        
Because the levels of C/N0 can vary and may reach very low levels, neither of the above methods is as reliable as desired. There is a need for methods and systems for obtaining improved bit transition detection.